1. Field of the Invention
The present invention relates to a sheet feeder for conveying and supplying with a printing sheet to a printing process and, more particularly, to a sheet feeder for a sheet-fed press which can feed a printing sheet accurately and readily without defective feeding.
2. DESCRIPTION OF THE PRIOR ART
A conventional sheet feeder for a sheet-fed press will be outlined in accordance with FIG. 1A. A bundle 2 of printing sheets comprising a lot of printing sheets 27 piled up is placed on a loading base 9. These printing sheets 27 are successively conveyed, one by one from the upper end of the bundle 2, to a printing process, where they are subjected to a prearranged printing process.
It sometimes occurs that a plurality of printing sheets 27 are made to adhere due to electrostatic force or the like. Such a case may result in feeding two or more printing sheets 27, obstructing the subsequent printing process. In order to prevent such two sheets feeding and feed exactly only one sheet to the printing process, the following way has been adopted.
As shown in the figure, an injection nozzle 6 is provided near the upper side edge of the bundle 2 to thereby jet air toward the bundle 2 of printing sheets. Owing to jets of air, several to several tens of printing sheets 27 constituting the upper part of the bundle 2 are forced to float and separate from the remaining part of the bundle 2, thus separate printing sheets 10 are formed. As described above, by separating printing sheets 27 into independent pieces, it is possible to relieve two sheets feeding due to electrostatic force or the like. Further, a sheet separator 12 is attached on top of the injection nozzle 6, allowing a top sheet of separate printing sheets 10 to be caught thereby. This serves to set limits to the floating height of printing sheets 27, so then the top printing sheet 27 is kept in a fixed position.
In addition, a paper pressure bar 4 for applying pressure on printing sheets 27 is provided on top of the bundle 2. Provision of the paper pressure bar 4 is intended to apply pressure on printing sheets 27 in the width direction from side to side and thereby block jets of air. By the presence of the paper pressure bar 4, it is possible to efficiently send the air from the injection nozzle 6 into every spaces of each adjoining printing sheets 27 and form separate printing sheets 10. Incidentally, the paper pressure bar 4 is unrestrictedly movable in directions of arrows 93 and 94. Also, at every moment when the printing sheet 27 is conveyed, the paper pressure bar 4 periodically rises in the direction of an arrow 95 so as not to impede sheet conveyance.
An absorption foot 8 is provided close over separate printing sheets 10, as shown in the figure. First, the absorption foot 8 lowers in the direction of an arrow 92 and holds the top printing sheet 27 of separate printing sheets 10 by absorbing it. Then, the absorption foot 8 rises in the direction of an arrow 91 and thereafter moves in the direction of the arrow 93, thus conveying the printing sheet 27 to the prearranged printing process. Incidentally, the loading base 9 is made to lift according as printing sheets 27 are fed to decrease.
If the top printing sheet 27 of separate printing sheets 10 is not floated up to the position of the sheet separator 12, the absorption foot 8 cannot absorb the printing sheet 27. Further, even in the case where the top printing sheet 27 is extended to the sheet separator 12, if too many printing sheets 27 are made to float up, floating sheets are closed to each other and adhered due to electrostatic force or the like. This is responsible for two sheets feeding. Therefore, it is desired as optimum separation state that the top printing sheet 27 is extended to the position of the sheet separator 12 with every floating sheets moderately separated.
However, the optimum separation state becomes different according to sheet thickness, sheet quality or the like. Consequently, in sheet feeding, it has been necessary to establish the optimum separation state in compliance with the printing sheet 27 involved. The optimum separation state is established by adjusting the injection air quantity from the injection nozzle 6 or by moving the paper pressure bar 4 in directions of arrows 93 and 94, with the state of separation visually inspected at the same time.
However, the conventional sheet feeder for a sheet-fed press has the following problems. Establishing the optimum separation state of the printing sheet 27 to be processed is conducted by adjusting injection air quantity or by moving the paper pressure bar 4 with the aid of manual operation of a worker. Printing sheets 27 are allowed to float up to higher position by increasing the injection air quantity from the injection nozzle 6, whereas the floating height is made to lower when the injection air quantity from the injection nozzle 6 is decreased. Also, moving the paper pressure bar 4 in the direction of the arrow 94 causes air to be jetted over a wide range of each printing sheet 27, with the result that printing sheets 27 are floated up to higher position. On the other hand, if it is moved in the direction of the arrow 93, the floating height of printing sheets 27 becomes low.
As described above, by adjusting injection air quantity, moving the paper pressure bar 4, or combining these two operations while visually inspecting the state of separation at the same time, a worker, through trial and error, establishes the optimum separation state. The operation of adjustment, therefore, takes a lot of time and also requires a skill, leading to the problem that the optimum separation state is not readily established.
In addition, even if the optimum separation state is established before starting sheet feeding as described above, it sometimes turns ill-suited in the course of sheet feeding because of the change in printing speed in printing or the rise of the loading base 9. As a result, the problem of defective sheet feeding such as feeding two printing sheets 27 or the like may occur.
An additional problem is as follows. As shown in FIG. 1B, printing sheets 27 sometimes curl due to, for example, sheet property, or the effect of printing ink parched after they are subjected to printing. In such a case, the absorption foot 8 cannot securely absorb the printing sheet 27, because the printing sheet 27 and an absorption surface 8Q of the absorption foot 8 are not placed in parallel with one another. Correcting the position of such printing sheets 27 is more difficult as compared with ordinary adjustment.